The present invention relates to a cryogenic heat pipe having a sealed tube and designed to cool an object (to be cooled) placed on the heat absorbing side by using a refrigerator installed on the heat dissipating side and, more particularly, to a cryogenic heat pipe which can efficiently perform heat transport by properly setting the inner diameter of the above tube and the length of a heat absorbing portion.
Superconductors are known to have zero electric resistance. It is expected that the energy consumed by power equipment will be saved by applying this property to power equipment.
In order to use a superconductor, it must be cooled to a critical temperature or less by some means. As a cooling means, a scheme of cooling a superconductor by immersing it in a cryogenic liquid such as liquid helium or liquid nitrogen is generally used. With such an immersion dip cooling scheme, however, since liquid helium or liquid nitrogen which is difficult to handle is used, it is inevitable that the operation cost will rise.
Under the circumstances, a direct refrigerator cooling scheme has recently been proposed, in which the cooling stage of a refrigerator is thermally connected to a superconductor through a heat conduction member to cool the superconductor.
In power superconducting equipments, however, since large currents are involved and AC is used, a larger amount of heat is generated than in DC superconducting equipment. In addition, in the power superconducting equipment, a sufficient distance must be ensured between the refrigerator and the object to be cooled owing to the large size of the equipment and the breakdown voltage. For these reasons, a large amount of heat needs to be transferred over a long distance. This makes the temperature difference between the refrigerator and the object large, resulting in a considerable deterioration in the efficiency of the system.
Demands have therefore arisen for the development of heat transfer elements for transferring heat with a small temperature difference over a long distance. Of these elements, elements for actively transferring heat by using the movement of a fluid flowing through a heat pipe, a dream pipe, or the like are especially expected to be developed. Of these elements, especially a cryogenic loop heat pipe having a loop capillary tube has advantages. For example, this pipe has good operability because it requires no special fluid driving source, and also exhibits a high degree of freedom in installation because it can have a flexible structure as a whole.
The structure of the cryogenic loop heat pipe will be briefly described below.
The cryogenic heat pipe is formed by sealing a working fluid into a capillary tube consisting of copper and shaped into a loop.
When the loop capillary tube is to be actually used as the heat pipe, a portion of the tube is thermally connected, as a heat absorbing portion, to an object to be cooled, and the other end of the capillary tube is thermally connected, as a heat dissipating portion, to a heat absorption object, e.g., a cooling source. In many cases, these portions are connected by using blocks or the like consisting of a good heat conduction material.
When the working fluid in the heat absorbing area is heated by heat entering the heat absorbing portion, a vapor bubble is generated in the working fluid. This vapor bubble displaces the liquid around the bubble. Although this displacement force acts toward the two sides of the loop with the heat absorbing portion serving as a boundary, the force in one direction can be made stronger by, for example, finely unbalancing the arrangement. As a result, the flow component of the liquid in one direction increases, and the working fluid circulates or oscillates in the loop. This movement of the working fluid contributes to heat exchange between the heat absorbing portion and the heat dissipating portion, thus performing heat transfer. Since the latent heat of evaporation is absorbed when the fluid is evaporated at the heat absorbing portion and releases it when gas condenses at the heat dissipating portion, at a small temperature difference, a large amount of heat can be transferred in particular. For this reason, heat can be transferred in an amount 10 to 100 times that transferred by using a copper member, as a heat transfer element, which has the same cross-sectional area as that of the capillary tube.
Although the cryogenic loop heat pipe has such advantageous characteristics, the pipe does not operate if the dimensions of the capillary tube fall outside the operating conditions. For this reason, a cryogenic loop heat pipe must be designed after a sufficient examination. For example, the inner diameter of the capillary tube is empirically determined by trial and error. In addition, when the operating temperature range is determined, the type of operating fluid that is suited for this temperature range must be used. However, since optimal capillary tube inner diameter varies from one working fluid to the other, the same trial-and-error testing must be repeated.
As described above, in order to design a cryogenic heat pipe, a test must be performed under each operating condition. Currently, this problem interferes with the applications of the cryogenic loop heat pipe.